IgG subclass restriction of autoantibody to solid-phase C1q in membranoproliferative and lupus glomerulonephritis

The IgG subclass distribution for autoantibodies to solid-phase C1q (anti-spC1q) in sera from 14 patients with membranoproliferative glomerulonephritis (MPGN) and 10 patients with systemic lupus erythematosus (SLE) nephritis was determined by an enzyme-linked immunosorbant assay employing C1q as the immunosorbant in the presence of 2 M NaCl to prevent Fc binding and monoclonal anti-human IgG subclass reagents. The autoantibody to spC1q in MPGN, especially in types I (7 patients) and II (3 patients), was almost entirely restricted to IgG3. In contrast, in SLE anti-spC1q was completely restricted to IgG2 in 3 patients while predominantly IgG2 in the other 7 patients. The different subclass restriction of anti-spC1q in these two disorders suggests that antibody formation is either in response to different epitopes on the collagen-like region of C1q or that patients with SLE and MPGN mount different immunologic responses to the same antigenic stimulus.